[1]} proposed that the O i \(\) 1304/\(\) 8446 ratio is another useful reddening indicator since almost every \(\) 8446 transition will be followed directly by a \(\) 1304 transition. The number of \(\) 1304 photons is the same as the number of \(\) 8446 photons but the intensity of the \(\) 8446 line is stronger because a \(\) 1304 photon has more energy than a \(\) 8446 one. This thus gives an O i \(\) 1304/\(\) 8446 intensity ratio of 8446/1304 = 6.48. [2]} and [3]} noted minor effects that might possibly change the \(\) 1304/\(\) 8446 ratio (see [3]}, section IId). Various factors which might affect the O i \(\) 1304/\(\) 8446 ratio were also discussed by [5]} and modelled by [6]}. The latter concluded that the ratio was close to 6.48 for high-column-density clouds with densities and ionization parameters similar to those expected for the BLR. The situation in real BLRs will be even better because observations imply that BLR emission is spread out and highly radially stratified by ionization [7]}, [8]}, rather than coming from high-column-density clouds where each cloud emits all lines. Furthermore, photons will escape from the sides of clouds (see Figure 3 of [9]}). We therefore expect the unreddened theoretical O i \(\) 1304/\(\) 8446 ratio to be close to 6.48.
